This invention relates in general to wireless communication systems, and more specifically to a method and apparatus for facilitating an estimation of a carrier frequency error in a receiver of a wireless communication system, that mitigates effects of Doppler frequency shift.
Some applications, such as the low-power side of an asymmetrical two-way messaging system, require high-sensitivity receivers. To conserve battery power in the portable subscriber units, such systems utilize low-power transmitters for the inbound (portable-to-base) links. To help rebalance the inbound and outbound link gains, extensive measures are taken to maximize the sensitivity of the base receiver.
One such measure is to utilize a multi-symbol matched filter (MSMF) receiver as the base receiver. The MSMF receiver provides excellent sensitivity. Prior-art MSMF receivers, however, have lost sensitivity when the reception environment includes time-variant Doppler frequency shift. Such frequency shift can occur when a portable subscriber unit is moving rapidly, as when in a motor vehicle. The loss of sensitivity occurs because the carrier acquisition techniques of the prior-art MSMF receivers have not been able to track the Doppler frequency shift adequately.
Thus, what is needed is a method and apparatus for facilitating an estimation of a carrier frequency error in a receiver of a wireless communication system, that mitigates the effects of Doppler frequency shift.
An aspect of the present invention is a method for facilitating an estimation of a carrier frequency error in a receiver of a wireless communication system, that mitigates effects of Doppler frequency shift. The method comprises the steps of receiving a first signal comprising a sync portion including a carrier acquisition segment and a timing synchronization pattern, and calculating a first plurality of squared magnitude Fourier transforms at a first plurality of frequencies on the carrier acquisition segment. The method further comprises the steps of deriving an initial carrier frequency error estimate by locating a peak in the first plurality of squared magnitude Fourier transforms, and correcting the sync portion according to the initial carrier frequency error estimate, thereby producing a carrier-corrected sync portion. The method further comprises the step of removing the timing synchronization pattern from the carrier-corrected sync portion, thereby producing a second signal having a residual carrier error.
Another aspect of the present invention is a receiver in a wireless communication system for facilitating an estimation of a carrier frequency error that mitigates effects of Doppler frequency shift. The receiver comprises a receiver element for receiving a first signal comprising a sync portion including a carrier acquisition segment and a timing synchronization pattern, and a processing system coupled to the receiver element for processing the sync portion. The processing system is programmed to calculate a first plurality of squared magnitude Fourier transforms at a first plurality of frequencies on the carrier acquisition segment, and to derive an initial carrier frequency error estimate by locating a peak in the first plurality of squared magnitude Fourier transforms. The processing system is further programmed to correct the sync portion according to the initial carrier frequency error estimate, thereby producing a carrier-corrected sync portion, and to remove the timing synchronization pattern from the carrier-corrected sync portion, thereby producing a second signal having a residual carrier error.